ion: simplify client name uniquification

The current method of finding a unique serial number for Ion client
names is not straightforward and prone to future breakage. Rather than
parsing all client names and extracting the max out of that just store
the client `id' in a new field in `struct ion_client'.

Change-Id: Ic25008e690ae4f9e3b4c19d1e438b8aaadb36a32
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>